Use SingletonThreadLocal for the current fiber manager

Summary: [Folly] Use SingletonThreadLocal for the current fiber manager, which does not depend on whether `__thread` or `thread_local` is available.

Reviewed By: andriigrynenko

Differential Revision: D19229511

fbshipit-source-id: a92016aca89205c2842dc4f71054b1b6306b14b9

folly/fibers/FiberManager.cpp

@@ -83,8 +83,8 @@ auto FiberManager::FrozenOptions::create(const Options& options) -> ssize_t {
 }
 
 /* static */ FiberManager*& FiberManager::getCurrentFiberManager() {
-  static FOLLY_TLS FiberManager* currentFiberManager;
-  return currentFiberManager;
+  struct Tag {};
+  return SingletonThreadLocal<FiberManager*, Tag>::get();
 }
 
 FiberManager::FiberManager(